The present invention relates generally to imaging systems, and specifically to Fresnel zone imaging methods in ultrasound imaging systems.
Ultrasound imaging is usually achieved through the use of a transducer array, which is composed of multiple individually addressable elements that are activated in such a way as to form an acoustic beam (beamforming). The beam is of a certain shape and is directed towards specific locations in the imaging medium that suit the particular imaging configuration. The transducer array receives back scattered signals from the imaging medium which are then used to create an ultrasound image.
In most imaging applications, image quality is an important parameter. Typically, image quality metrics include spatial resolution and contrast resolution. Image quality, including resolution and contrast, is directly related to the acoustic beam characteristics. Characteristics of the acoustic beam are in turn determined by the physical properties of the transducer array, the imager's transmit and receive electronics, as well as the electronic phasing or time delays applied to the array elements.
It is desirable to confine the acoustic beam to as small a spatial region as possible. Sidelobes are regions around the main beam where significant acoustic energy is both propagated and sensed. Sidelobes are usually undesirable as they reduce both image resolution and contrast.
Specifically, in Fresnel zone imaging, a transducer can be divided into regions or zones that contribute either constructively or destructively to the focus point, based on the geometric propagation distance between the focus point and the specific point on the transducer being considered. The Fresnel zones simplify the system hardware in two ways. Firstly, it allows larger groupings of elements to use a single time delay. In other words, all the elements within a given zone share the same delay rather than independent time delays for each element. Secondly, especially in optical or similar narrow-band systems, one specific time delay for all constructive zones is used and a different time delay is used for all destructive zones.
In Fresnel zone imaging with cMUT or electrostrictive elements, regions of destructive interference can be converted to regions of constructive interference by applying an apodization profile consisting of discrete, relative apodization weights of −1 and +1 to the destructive and constructive regions respectively. One problem with the above described method in which the weighting of the various regions of the transducer is restricted to the values +1 and −1 is the generation of radiation patterns with a certain sidelobe level.
Accordingly, there is a need to generate a weighting pattern that minimizes the generation of sidelobes while using Fresnel zone imaging techniques.